This invention relates to thermostatic electrical switches and more particularly to low cost, compact, snap-acting thermostatic switches and their method of assembly.
The manufacturing of thermostatic switches employing transfer pins or the like has heretofore been beset with difficulties in producing a compact, low cost, reliable switch. A main reason for the difficulties is the fact it is of the utmost importance that calibration of these switches be precise and reliable. Typically the travel of a snap-acting disc in these thermostats is in the order of 0.010 of an inch. This relatively small travel movement of the disc coupled with the variations in sizes of transfer pins and other components has necessitated complex designs and assembly procedures to insure proper calibration.
Accordingly, it is an object of this invention to provide an improved, compact, low cost thermostatic switch.
It is another object of this invention to provide a thermostatic switch which can be automatically assembled and calibrated.
It is still another object of this invention to provide a thermostatic switch in which calibration, in at least one of the embodiments, can be effected externally of the switch after assembly of the parts therein as a final operation.
It is a further object of this invention to provide an improved method of assembling and calibrating the thermostatic switch. Other objects will be in part apparent and in part pointed out hereinafter.
Briefly, the switch made according to the present invention comprises a single housing member, a contact terminal, a movable arm terminal, two rivets, two electrical contacts, a movable contact arm with a transfer pin attached thereto and a snap-acting bimetallic strip disc. The housing member has a central aperture therethrough with a larger opening at the top of the housing than at the bottom forming a shoulder near the bottom of the housing. On either side of the central aperture is a recessed circular aperture for receiving one of the rivets. One of the electrical contacts is attached to the contact terminal and the other is attached at the distal end of the movable contact arm.
The bimetallic strip disc member is inserted through the top central aperture to rest unrestrained on the shoulder near the bottom of the housing. Then the contact terminal with the contact attached is secured in place by one of the rivets. Next the movable arm terminal and movable contact arm with contact and transfer pin attached are secured to the housing with the other rivet over the central aperture. The rivets secure the two contacting means so that the two electrical contacts, one on the contact terminal and the other on the movable contact arm are initially in engagement with each other. The device is fully assembled and ready for calibration.
The calibration is done with the help of a strain gauge fixture. The fixture is set for the type and shape of the bimetallic disc used and positioned so as to be in contact with the disc to accurately read out contact forces so proper calibration can be performed. All the calibration adjustments are done from the top in assembled form. To help provide for easy calibration the movable contact arm has two right angle bends at the distal end between the transfer pin and electrical contact. The two right angle bends make the movable arm more rigid so there is no lost motion between the pin and the contacts. With this switch the size can be kept small and the construction inexpensive and the various components need only be of approximately correct size.